The flooding of a village 40 years ago to provide water for Liverpool was not necessary, according to new research.
70 people were driven from their homes to provide the reservoir
Drowning a Village, screened on BBC2 Wales, claims Liverpool Corporation had more than enough water when the Tryweryn reservoir was created.
Seventy people were forced to move from welsh-speaking Capel Celyn near Bala in 1965 after nine years of angry protest.
In 2005 Liverpool City Council apologised for the "insensitivity" of its predecessor council at that time.
Every Welsh MP apart from one had voted against the scheme.
But Liverpool Corporation, with the backing of English MPs, had driven through a parliamentary bill that would see the flooding of an 800-acre valley and cause the destruction of a school, a post office, a chapel and cemetery. It flooded 12 farms and destroyed land owned by four others.
The protests had been angry and widespread, winning support from all political parties in Wales.
And when dignitaries gathered for the official opening of the Tryweryn reservoir in 1965 the party was stopped as when protestors climbed over the dam.
The issue is credited with creating the wider nationalist movement and brought support for Plaid Cymru, which soon after won its first elected political representation.
Programme producer Llion Iwan, also a lecturer in journalism at the University of Wales in Bangor, said working on the project opened his eyes to the turbulent times that lead up to the flooding.
The protests lead to a rise in support for Welsh nationalism
"The research I did shocked me," he said, "strongly suggesting that it was never necessary to drown the village at all.
"Even today, Liverpool does not use all the water from Tryweryn.
"The questions haven't been fully answered - for example, what use is made of the 'excess' water, if you can call it that?"
The city also has reservoirs at Vyrnwy and Rivington.
Mr Iwan said he was motivated to make the programme because of his personal connection to the area - his grandfather was on the Capel Celyn defence committee.
"I thought I knew everything about this episode in our history, but talking to people who lived in the area at the time brought the story alive for me," he added.
The Welsh-speaking village of Capel Celyn was lost in the flooding
"Discovering the photographs in the archive that record raising the cemetery was startling as well, these images are sure to hit the message home to the viewers."
The programme uses music written by the award-winning composer Guto Puw. Mr Puw's grandfather Dan lost farmland to the reservoir.
Drowning a Village is on BBC2 Wales on Thursday at 1930 GMT.